The carbon lining which forms the internal, side and bottom walls of an electrolytic reduction cell gradually degrades due to the extremely high temperatures and the corrosive conditions that exist during operation of the cell. This degradation gradually causes failure of the carbon blocks which make up the cell, allowing molten aluminium to penetrate the carbon blocks which often causes distortion of the cell. At this stage the cell is removed from service in the pot line. In addition to the carbonaceous cathode material, the refractory and insulating materials that surround the cell as well as the steel cathode bars imbedded in the bottom of the cell are also removed. This material is called spent pot lining or SPL.
During its life the carbon lining of the electrolysis cell absorbs quantities of the bath materials which include aluminium metal, sodium aluminium fluorides and other fluorides. Aluminium carbides and nitrides are also formed during cell operation and these are also deposited in the carbonaceous cathode material. Spent pot lining is currently listed as a hazardous waste by the U.S. Environmental Protection Agency as it contains potentially harmful leachable cyanides and fluorides that can enter the ground water during open air storage. As well, ammonia, hydrogen, hydrogen cyanide, methane and phosphine are produced when the material becomes wet. Various disposal techniques where the SPL can either be destroyed or the materials in the SPL used in other industrial processes have been developed over the years. However, of these processes none has been accepted as standard practice in the industry.
These include processes where the spent pot lining can be used as a replacement for fluorspar in the steel industry or where the carbon values of the spent pot lining are used as a supplementary source of fuel. For example spent pot lining has been burnt in cement kilns and here the cyanide is destroyed while both the carbonaceous material and the fluoride values are used.
Although fluidized bed combustion techniques for disposing a spent pot lining has been demonstrated on a pilot scale, this technique has yet to be demonstrated on a commercial scale. While the cyanide levels of the spent pot lining have been reduced to acceptable levels in such instances, the ash still contains the fluorides which have to be immobilized for example with calcium hydroxide, if the ash is to be disposed as landfill.
As the spent pot lining contains significant amounts of fluorine containing chemicals as well as appreciable amounts of aluminium which can be recycled into the aluminium smelting process, there is an economic incentive both for recovering these values and for producing a spent pot lining residue which can be disposed in an environmentally acceptable manner.
Examples of processes aimed at recovering aluminium fluoride from spent pot lining materials are to be found in U.S. Pat. Nos. 4,508,689 Bush et al, 4,597,953 Bush and 4,889,695 Bush. In these processes, the pot lining material is crushed and leached to extract the fluoride and aluminium values. However, the processes described in the above patents do not address the problem of disposal of the potentially dangerous cyanide containing residue and the fluoride and aluminium value recovery rate is not particularly high.
A membrane process for treating SPL has been developed which uses low temperature solution processing using the well known cryolite recovery technology. However, the solution still contains silica, aluminium, iron and cyanide and these components can lead to membrane fouling.